The isolation of the lumenal plasma membrane from the mammalian urinary bladder is being carried out by detachment of the surface membrane resulting from its exposure to thioglycolate and purification by centrifugation on a discontinuous sucrose gradient. Improvement in purity and yield of this procedure, previously described by Chlapowski et al. (J. Cell Biol. 53:97), is in progress with the goal of achieving a biochemical analysis of the lipids present (which may account for the impermeability of the membrane to water) and of the characteristic protein particles that are assembled into plaques within the membrane. The particles, once isolated in pure form, will be used as antigens for immunochemical staining of bladder epithelium in order to determine whether they may be assembled and integrated into membrane within the Golgi region.